A large group of supporters surrounded Koeman as he attempted to leave Camp Nou, shouting insults at him and striking his vehicle, with Barca later branding the incident "violent and disdainful."
"I was with my wife, but there were also people behind us filming [a documentary], so I have everything recorded," Koeman said in a news conference on Tuesday. "There was that one guy wearing the Arsenal shirt behind the car, you saw him, right?
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"I wasn't scared. There was a moment I thought about getting out, but it was better not to. There were a lot of people filming and [making] Tik-Tok [videos] and they want you to get involved."
Barca have recently encountered problems with people trying to get a rise out of players for Tik-Tok videos at their training ground. ESPN reported in March that they were looking into increasing security due to the constant filming.
However, Koeman, who was not the only one targeted after the 2-1 Clasico loss at the weekend, said it is very difficult to solve the issue.
"I don't think there is a solution," he added. "For me, it's more of a social problem, an education problem for people who don't have morals or values.
"The fans in the stadium were the opposite, even at 2-0 down, so we should not focus too much on these people. We have it here every day coming in and out [of the training ground].
"It was not only me it happened to, but a lot of players, too, who were with their families. It happened to everyone, even [former Barca defender] Carles Puyol."
Frenkie de Jong will miss the fixture with a hamstring strain, joining Ronald Araujo, Pedri, Martin Braithwaite and Ousmane Dembele on the sidelines for the trip to Madrid with Ansu Fati also a doubt.
Koeman explained the forward has a minor problem -- which he stressed was unrelated to the knee injury which kept him out for 10 months -- and a decision on his availability will be made on the day of the game.
The match comes too soon for Dembele, who returned to training with his teammates on Monday and could feature on Saturday when Barca host Alaves.
ESPN revealed on Monday that the France international's involvement moving forward, though, will be dependent on whether he is willing to sign a contract extension, with his existing terms up next summer.
Sources said the Catalan club will banish Dembele, who has drawn interest from England, to the stands if he doesn't sign a new deal, a tactic deployed with Ilaix Moriba in the summer but not one Koeman would confirm.
"The most important thing is that Ousmane stays," Koeman added. "He's an important player with different qualities in attack. The main objective is to renew his contract.
"It's another issue if he doesn't want to renew. We need to talk as a club about the best way to deal with that. We have still not decided what our strategy will be in the event he does not renew."